I think the reason $HTTP_XXX_VARS are used is because of people using lower versions of PHP where the newer version of super globals will not be recognised.

If you look at the Version 3 development page (http://www.phpee.com/pphlogger3-dev/) You will notice that the only versions of PHP that will be supported are 4.3.0 and greater.
I think we can safely say that the newer super globals will be used instead of the old style

Right, $HTTP_XXX_VARS are deprecated. PPhlogger 2.x actually should still run under PHP3 (AFAIK >= 3.0.9). There won't be any major changes in its code design as I took up development on PPhlogger3 again. As announced on http://www.phpee.com/pphlogger3-dev/, PPhlogger3 will only support PHP >= 4.3.0. Wrong, even here I have to correct myself: PPhlogger3 will ONLY support PHP 5! Currently PHP 5.0.0b4 is out and the final release is expected this summer. At this time I'm not going to explain why I made this decision. It's still not 100% sure, but there is a high chance that I'll drop all PHP4 support in this future version.
This is the first public announcement about this fact. I'm going to post about that in the announcement board as soon as I got more details.